1. Field of the Invention
This invention relates to cement compositions for the oil industry and a method for their use in cementing wells. In another aspect, this invention relates to the control of the gas migration or gas channeling phenomenon that occurs during the setting of a cement slurry.
2. Description of the Related Art
Gas channeling is a phenomenon that occurs during the setting of a cement slurry. Once the cement slurry begins to set, the hydrostatic pressure in the cement column begins to decrease. This reduction in hydrostatic pressure allows the channeling of gas through the partially set slurry. This gas, which is under pressure from the formation, flows through the partially set cement slurry creating microchannels. These microchannels can lead to safety problems when they extend to the surface.
One method used in an attempt to control gas channeling has been to include a styrene-butadiene latex in the cement. U.S. Pat. No. 4,537,918 describes a cement composition capable of inhibiting pressure gas channeling in which the slurry consists of cement, 5-30 weight % styrene/butadiene (30-70 weight %) copolymer latex, a latex stabilizer and water. However, this cement composition requires a minimum of 5% by weight of the cement of the styrene - butadiene latex creating problems of cost effectiveness. Thus, it is desirable to devise a cement composition that both prevents gas channeling and is more cost effective than known methods.